PPP, PPPoS, check if PPPoS configuration functions are used on PPPoS

We don't have callbacks which can be set or cleared for PPPoS
configuration, there is too much callbacks to create and PPPoS must be
kept light, therefore PPPoS functions can be called when PPP core
configure a PPPoE or PPPoL2TP interface, this is very unlikely to
happens because protocols not supported by PPPoE or PPPoL2TP are
disabled at LCP/IPCP negotiation but being safe is still better.

Check if passed PPP pointer to PPPoS configuration functions is a PPPoS
interface by using a linked list of exiting PPPoS interfaces.

src/include/netif/ppp/pppos.h

@@ -45,6 +45,7 @@
  */
 typedef struct pppos_pcb_s pppos_pcb;
 struct pppos_pcb_s {
+  pppos_pcb *next;
   ppp_pcb *ppp;                  /* PPP PCB */
   sio_fd_t fd;                   /* File device ID of port. */
 #if VJ_SUPPORT